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2006\01\31@221333 by

Hi All,

I have a small AC adapter, with the following spec. printed on a label
attached to it:-
Class 2 Transformer
INPUT: 120V AC 60Hz
OUTPUT: 12V AC 200mA

I tested this AC adapter by connecting a 50 ohm resistor to the output and
use an oscilloscope to read the output signal. I found that the signal is a
sine wave at about 30V p-p. I then removed the resistor and read the output
with NO load, the signal is a sine wave at about 40V p-p.

The spec. as marked on the adapter is "12V AC", how can we have an output
that high? Is the manufacturer cheating? BTW, what is "Class 2 Transformer"?

John

John,
40V pk-pk is equiv to 20V pk-0 which works out to about 14.4V rms. for
a sinewave.
So it's maybe a little high.

As an alternative - think of your 110V mains. This is 110V "rms" or
equivelent power to a 110V DC supply. So over a 1/2 cycle the power
would be the same. Therefore the peak voltage for a sinewave would be
sqrt(2)*110 = 155.5V. Peak-peak is twice this or 310V.

RP

On 01/02/06, John Waters <john_fm_watershotmail.com> wrote:
{Quote hidden}

> -
On Tue, 2006-01-31 at 19:13 -0800, John Waters wrote:
> Hi All,
>
> I have a small AC adapter, with the following spec. printed on a label
> attached to it:-
> Class 2 Transformer
> INPUT: 120V AC 60Hz
> OUTPUT: 12V AC 200mA
>
> I tested this AC adapter by connecting a 50 ohm resistor to the output and
> use an oscilloscope to read the output signal. I found that the signal is a
> sine wave at about 30V p-p. I then removed the resistor and read the output
> with NO load, the signal is a sine wave at about 40V p-p.
>
> The spec. as marked on the adapter is "12V AC", how can we have an output
> that high? Is the manufacturer cheating? BTW, what is "Class 2 Transformer"?

Remember, when dealing with AC there are SEVERAL ways of measuring
things. The 12V speced is the RMS voltage.

For a pure sine way the RMS voltage is the peak voltage divided by the
square root of two.

The peak voltage is half the peak to peak voltage.

So, 12V RMS -> 12*1.41 = 16.92V Peak -> 16.92*2 = 33.82V Peak to Peak.

So, your adapter is pretty close to spec, as can be expected with that
sort of device.

Unloaded it's normal for a wall wart like that to output higher then
spec.

As for Class 2, I don't remember off hand, I'm sure you can look it up
on google, but gut feeling is it has to do with the insulation and
failure modes (i.e. it won't get hot enough to catch fire if shorted).

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

2006\02\01@025238 by
Herb explains it perfectly. The 50 ohm resistor loaded the output and
it dropped appropriately.  Adjust that resistor until you measure 200mA
of AC current through it, and the output from the transformer will
measure 12VAC RMS.

Incidentally, did you know that the way transformers are now made  has not
changed appreciably from the way Nikola Tesla designed them in 1890?

--Bob

Herbert Graf wrote:

{Quote hidden}

--
Note: To protect our network,
attachments must be sent to
attachengineer.cotse.net .
http://beam.to/azengineer

> Adjust that resistor until you measure 200mA of AC current
> through it, and the output from the transformer will measure
> 12VAC RMS

I bought a pile that are "12VDC 200mA". They are actually
resistor (470R) across the supply to take the sting out of it.
It does put out the advertised 12VDC (ripply, believe it or
not) at 200mA, which is what is wanted for driving an electric
bell, and relay

A plugpack is something you should ALWAYS ALWAYS
measure before you use it as a supply, especially if it's unregulated.

"6VDC 500mA" might be 10V or more unloaded and you could
do a lot of damage to a circuit that loads it too lightly. Add to the
label if you want to, for future reference. One day you might need
a 10V 50mA and that 6V 500mA could be it

Labels on unregulated plugpacks are at best sloppy and misleading
if you don't know that the stated output voltage is generally at about
the stated drain. If you're lucky and the manufacturer isn't too over-
enthusiastic about the capabilities of his product

At 12:52 AM 2/1/2006 -0700, you wrote:
>Herb explains it perfectly. The 50 ohm resistor loaded the output and
>it dropped appropriately.  Adjust that resistor until you measure 200mA
>of AC current through it, and the output from the transformer will
>measure 12VAC RMS.
>
>Incidentally, did you know that the way transformers are now made  has not
>changed appreciably from the way Nikola Tesla designed them in 1890?
>
>--Bob

Tesla used a sweatshop in China?

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff

On Wed, 2006-02-01 at 21:32 +1300, Jinx wrote:
{Quote hidden}

As odd as this might sound, you should be HAPPY that is the case! :)

One of the primary reasons wall warts like that are so "bad" is due to
their safe design. They are designed to either current limit (rarer) or
burn out (most common) if shorted. The easiest way to do this is simply
to make the windings thin enough that they burn out if overloaded.
Unfortunately this means they have a rather high source impedance, so
the transformer ratios are adjusted so that, under load, and given the
source impedance, the output voltage is correct.

It's not that ideal a design, but it sure is cheap to make! :)

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

On Wed, 1 Feb 2006, Spehro Pefhany wrote:

{Quote hidden}

No, he copied Michael Faraday's work which was open sourced, and used by
William Stanley to perfect power transformers as we know them in 1886
... ;-) (Nikola Tesla is probably most famous for pushing Ruhmkorff
induction coils (1857) to the practical limits possible at the time)

There are several great history sites on the net, and new ones appear
all the time. This is a great way to learn about tehcnology and its
history (and to prevent one from making crazy patent claims):

http://www.hp-gramatke.net/history/index.htm

Peter

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